You could add an external temperature controller (Inkbirds are popular) to maintain the freezer at whatever temperature you want. Or, if you can't do that, you could carbonate at room temperature...consult the charts (or brewing software) to see what pressure you'd need.
If your force carbing it helps if it’s cold but you can carbonate at room temp. To answer your question the colder to freezing but not freezing the better to help the beer absorb the CO2, and I have good results with 48hrs.
What are you keeping it in after you carb it? If it's going in a fridge for serving, or a kegerator, just force carb it and then throw it in your serving vessel. Like Transam said, it doesn't have to be cold to carb...it just absorbs the CO2 more efficiently.
IIRC gas diffusion (including carbonation) happens faster at WARMER temperatures. Requires a higher pressure, and doesn't stay in solution as well so if you try to pour without chilling you're gonna get a mess of foam. But it should actually carb more efficiently when warm.